The Pokémon Onix does not evolve when it hits a certain level but can only evolve when it is traded between players while holding the Metal Coat item. When traded with the item, Onix will evolve into the Steelix Pokémon.
While Onix is considered to be a rock and ground Pokémon, its evolutionary form Steelix belongs to the steel and ground species type. An average Onix measures a length of 28 feet 10 inches and weighs 463 pounds. The default abilities of this Pokémon includes Rock Head, Sturdy and a hidden ability called Weak Armor. Onix derives its name from the word "onyx," which refers to a type of rock.